Ionization chamber for use with a mass spectrometer

ABSTRACT

A replaceable ionization chamber for a mass spectrometer comprises an ionization region defined by two parallel perforated membranes attached to concentric tubular electrodes which are separated by the ionization region. Two filaments and two electron focusing electrodes are symmetrically disposed about the periphery of the ionization region, and sample input ports are similarly disposed about the periphery. An ion focusing electrode is mounted at the output end of the chamber. The components of the ionization chamber are supported by a plurality of conductive rods which are, in turn, supported by an insulating ring. The ionization chamber mates with a socket having a plurality of receptacles for selected ones of the conductive rods and a tubular section containing heating and cooling elements.

United States ?atent 1191 Kruger et a1.

[ Mar. 27, 1973 s4 IONHZATION CHAMBER FOR USE 2,975,277 3/1961 Von Ardenne ..2s0 41.9 WITH A MASS SPECTROMETER 3,217,160 11 1965 Craig etal. ..2s0 41.9 [75 1 Inventors: ag gjf g 8 T Primary Examiner-William F. Lindquist umer 0S a o Attorney-Roland I. Griffin of Calif. [73] Assignee: Hewlett-Packard Company, Palo [57] ABSTRACT Altocahf' A replaceable ionization chamber for a mass spec- [22] Filed: Feb. 2, 1971 trometer comprises an ionization region defined by two parallel perforated membranes attached to con- [21] Appl' L910 centric tubular electrodes which are separated by the ionization region. Two filaments and two electron [52] US. Cl. ..250/41.9 SB, 313/63, 313/231 focusing electrodes are symmetrically disposed about [51] Int. Cl ..H01lj 39/34 the periphery of the ionization region, and sample [58] Field of Search....250/41.9 G, 41.9 SB, 41.9 TF; input ports are similarly disposed about the periphery. /6 3 An ion focusing electrode is mounted at the output end of the chamber. The components of the ionization [56] References Cited chamber are supported by a plurality of conductive rods which are, in turn, supported by an insulating UNITED STATES PATENTS ring. The ionization chamber mates with a socket hav- 3,307,033 2 1967 Vestal ..2s0 41.9 ng a pl rality f receptacles for selected ones of the 3,356,843 12/1967 McElligott conductive rods and a tubular section containing heat- 3,423,584 1/1969 Erickson ing and cooling elements. 3,155,826 11/1964 Peters 3,418,513 12/1968 Elliott ..250/41.9 X 8 Claims, 2 Drawing Figures I I I Y 7Q -1 58 vxm II I PATENTEDMARZ? ma 3. 723, 729

. WILLIAM P. KRUGER lgure 2 WILSON R.TURNER IONIZATION CHAMBER FOR USE WITH A MASS SPECTROMETER BACKGROUND AND SUMMARY OF THE INVENTION Ionization chambers in mass spectrometers are highly susceptible to contamination due to the function they perform. Because of the many deleterious effects of contamination, most ionization chambers require a high level of maintenance by highly skilled personnel. One of the principal forms of contamination is plating out of positive ions of the sample on the filament and nearby electrodes. Such plating alters the work function of the filament, usually degrading its ability to emit electrons and eventually causing it to fail. Plating on the focusing electrodes also degrades their performance which leads to reduced spectrometer performance. Cleaning an ionization chamber and replacing a filament is usually a several hour job for a skilled technician. Not only is the operation expensive due to the involved cleaning process, but usually the mass spectrometer is out of service.

The ionization chamber of the invention disclosed herein is small in size and easily replaceable compared with prior art devices. Due to its small size, the chamber can be inserted or removed through a common vacuum port and gate valve. In addition, because the ionization chamber of the present invention is constructed much like a miniature vacuum tube, it can easily be unplugged from a socket provided in the spectrometer and be replaced by a fresh one. The ionization chamber is designed to be inexpensive enough that a contaminated one can be discarded, eliminating the costly cleaning process. The total down-time involved in replacing the ionization chamber is on the order of a few minutes.

Ions are generated in an ionization region defined by two parallel, perforated membranes each attached to one of two concentric tubular electrodes. Gaseous samples to be ionized diffuse into the ionization region via ports, typically made of glass and situated around the periphery of the region. Sample molecules are bombarded by electrons emitted by one or more filaments located around the periphery of the ionization region. The electrons are focused by electrodes adjacent to the filaments. An additional electrode, concentric with the above-mentioned tubular eledtrodes, is used to focus the ions emanating from the ionization region. Each electrode and filament is supported by one or more support rods which are fastened in an insulating support ring. Some of these support rods serve as connector pins that plug into a socket provided in the mass spectrometer for the ionization chamber. The socket has, in addition to receptacles for the conductive rods, heating and cooling elements which fit inside one of the tubular electrodes for controlling the temperature of the chamber. The heating element is usually a resistance type heater encased in an electrically insulating heat conductor, and the cooling element is a heat exchanger through which a cold fluid or gas is passed. The cooling element may also be a thermoelectric device or a chamber in which a coolant is expanded.

When electrons are emitted into a region containing gas molecules, some of the molecules will be ionized by collisions with electrons. The ions generated will depend upon the energy of the electrons and whether there are single or multiple collisions between a molecule and electrons. Since it is desirable to have a predictable and consistent distribution of ionic species in the fragmentation pattern for mass spectrometry purposes, it is necessary to control electron energies and reduce multiple collisions. Ionization chamber efficiency depends on what fraction of the molecules introduced to the chamber collide once with an electron.

Electron energy can be controlled by controlling the potentials on the filament and the electron focusing electrodes. Single and multiple collision probabilities depend upon the geometry of the ionization region and the electron density in the region. The geometry of the ionization region in the ionization chamber of the present invention is conductive to high single collision probabilities and low multiple collision probabilities, as is discussed in greater detail in the discussion of the preferred embodiment. Due to the higher ionization efficiency of the present invention a smaller sample is required, consequently reducing the contamination rate. The reduced contamination rate results in less frequent changing of the ionization chamber, allowing more'effective utilization of the mass spectrometer. The purer fragmentation patterns generated by the ionization chamber disclosed herein make more reliable mass spectra available.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a longitudinal cutaway view of an ionization chamber and socket according to the preferred embodiment.

FIG. 2 shows a transverse cutaway view of the ionization chamber of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT typically has a volume of 0.001 to 0.002 cubic inch.

Sample inlets 22 And 22a having flanges 24 and 24a are glass tubes for directing a gaseous sample to be ionized into region 20. Filaments 26 and 26a, made of tungsten for example, are sections of a toroid situated around the periphery of region 20. Electron focusing electrodes 28 and 28a are also situated around the periphery of region 20 to focus the electrons from filaments 26 and 26a to form essentially a sheet of electrons in region 20. Elements I4, 22, 22a, 26, 26a, 28 and 2811 are each supported by one or more metallic support rods 30. For example, filament 26 is supported by rods 30a and 30b. Each of rods 30 is fastened in insulative support member 32. In addition to supplying mechanical support for the various electrodes, rods 30 also act as electrical leads for the electrodes. Shield 34 is a perforated metal disc to shadow support member 32 from conductive deposits or evaporated films in order to maintain a high electrical resistance among rods 30 and electrode 12 which is also fastened in support member 32.

When molecules are introduced into region 20, electrons emitted by filaments 26 or 26a bombard them and ionize them. Usually just one filament is heated at a time so that the other serves as a backup in case of failure, but both filaments are maintained at the same potential with respect to the other electrodes. Thus the electric field in region 20 is spatially symmetrical about center lines 36 and 61. An electron emitted by filament 26, for example, which passes through region 20 without colliding with a sample molecule will encounter the same fields which focused it, in the reverse order as it approaches filament 26a. Filament 26a and electrode 28a may thus act as a mirror to the electron.

As mentioned above it is desirable to maximize the probability of single collisions between a molecule and an electron and to minimize multiple collisions of a molecule with electrons. Single collision probability can be increased by increasing the electron density in region 20, but such density is usually limited by space charge depression of potential within the region. Space charge depression of potential can be minimized by minimizing the thickness of region 20, i.e., the distance between membranes 16 and 18. For example, with elements 12 and 14 0.250 inch in diameter region 20 may be made less than 0.040 inch thick. A thin ionization region also helps reduce the probability of multiple collisions. Potentials placed on electrodes 12 and 14, and therefore on membrance 16 and 18, accelerate ions, usually positive ones, out of the chamber (to the right in FIG. 1). Thus the narrower region 20 is made, the sooner an ion can be removed from the region before being hit by another electron. Once the ions are accelerated out of region 20, they are focused by a tubular metallic electrode 28 supported by support rod 30c.

lons generally in ionization chamber 10 exhibit a more nearly ideal fragmentation pattern than prior art ion chambers because of the reduction of the multiple collision probability and the sample inlet structure. Some molecules which may be analyzed in a mass spectrometer undergo a chemical change if they contact a metal surface before being ionized. In ionization chamber 10 sample inlets 22 and 22a are made of glass, which is chemically inert; and the sample molecules must pass through ionization region 20 before encountering any metal parts. The glass ports in conjunction with the electrically confined but gaseously unconfined ionization region effect improved performance in two ways. Firstly, sample molecules emerge from either glass port directly into the electron beams and have a high probability of being ionized prior to impact upon any metallic surface; if not ionized, the sample molecules rapidly diffuse out of the ionization region due to its small size and the presence of high-transmission membranes 16 and 18. As a consequence, the number of molecules re-entering the electron beam after metallic contact is minimized. This is in sharp contradistinction to prior art so-called tight" ionization chambers which confine the sample in metallic boxes to enhance sensitivity at the risk of sample decomposition from metallic contact. Alternatively, if membrance 16 is unperforated and adequately chilled, it can condense and hence trap sample vapors and in that manner prevent their re-entry into the ionization region. Secondly, any loss in sensitivity due to rapid diffusion of sample from the ionization region as previously mentioned is more than compensated for by the advantageous flow pattern of the sample. Immediately prior to its emergence into the ionization region, the sample is confined within a capillary system of which entrance port 22 or 22a is the terminus. Even very small samples thus generate substantial molecular densities within the capillary system. The jet of sample emerging into the electron beam tends to maintain the high molecular density long enough to constitute an effective high pressure in the region of ionization, resulting in a net gain of ion amperes per torr of pressure in the spectrometer proper.

Socket assembly 40 comprises receptacles 42 and a probe 44 fastened in an insulating support member 46 which in turn is fastened in a metallic cylinder 48. The outer sheath 45 of probe 44 is conductive and is in con; ductive contact with electrode 12, thus serving as an electrical connector for electrode 12. Probe 44 also includes a heating element 50 encased in a thermally conductive, electrically insulative matrix 52. Heating element 50 may be nichrome wire, for example, and matrix 52 may be glass, pyroceram or ceramic. Heating element 50 and matrix 52 are supported by tube 54 and tube 56 is supported within tube 54. One terminal of heating element 50 is attached to tube 54 and the other to wire 58 supported by support member 46. Heating element 50 may be used to heat ionization chamber 10 if desired by the operator of the mass spectrometer. Tubes 54 and 56 may be used to cool ionization chamber 10 by passing a cold fluid or gas through them. Alternatively, a plug having an orifice in it may be placed in end 60 of tube 56. Then when a coolant such as liquid freon is passed through tube 56 it will expand as it passes through the orifice and become a gas. This expansion will remove heat from ionization chamber 10. A third alternative structure for cooling ionization chamber 10 comprises a Peltier effect thermoelectric heat pump mounted within tube 54, tube 56 being eliminated. One purpose of cooling ionization chamber 10 is to condense out any sample molecules coming in contact with the metal surfaces of ionization chamber 10, thus reducing contamination is the mass spectrometer. A second purpose is to reduce the probability of thermal dissociation of sample molecules upon contact with metallic parts of the ionization chamber, with subsequent ionization of the dissociated sample.

Six of the rods 30 plug into receptacles 42 to make the necessary external connections to electrode 14, filaments 26 and 26a, electrodes 28 and 28a and electrode 38. Receptacles 42 are arranged asymmetrically on the circumference of a circle about tube 54. The asymmetrical arrangement ensures that the ionization chamber 10 will be plugged into socket 40 in the proper orientation. Tube 48 slides into a Y4 inch vacuum port in the mass spectrometer and through the bore of a gate valve to place ionization chamber 10 properly adjacent the analysis section of the spectrometer.

We claim:

1. An ionization chamber comprising:

a plurality of conductive members;

a first and second coaxial, tubular electrode, the second tubular electrode being connected to at least one of said conductive members and each of the tubular electrodes having an electrically conductive membrance across one end, said ends being adjacent;

an ionization region situated between the electrically conductive membranes and within a perimeter of one of the tubular electrodes;

sample insertion means for introducing a sample to be ionized between said membranes;

an arcuate filament connected to a plurality of said conductive members and disposed exterior said ionization region and in a plane lying intermediate said membranes;

an electron focusing electrode connected to at least one of said conductive members, situated exterior said ionization region, and adjacent said filament for focusing electrons emitted by said filament into the ionization region for ionizing said sample;

a third tubular electrode coaxial with the second electrode and connected to at least one of said conductive members for coacting with the second tubular electrode to form a lens for focusing ions exiting from the ionization region;

an insulative support member for supporting said plurality of conductive members;

means for supporting the first, second and third tubular electrodes, the filament, and the electron focusing electrode; and 7 means for applying external signals via said conductive members to the filament and electrodes for causing electrons to bombard and ionize molecules of said sample and for propelling resulting ions through the electrically conductive membrance across one end of the second tubular electrode.

2. An ionization chamber as in claim 1 wherein:

a first and second arcuate filament are disposed opposite each other across a diameter of the ionization region and a first and a second electron focusing electrode are disposed adjacent said first and second filaments respectively;

the means for supporting the second and third tubular electrodes, the first and second filaments, and the electron focusing electrodes are said conductive members; and

said means for applying external signals includes socket means for engaging selected ones of said conductive members and said first tubular electrode.

3. An ionization chamber as in claim 2 wherein said socket means includes heating and cooling means in a portion engaging said first tubular electrode for heating and cooling said chamber.

4. An ionization chamber as in claim 3 wherein said cooling means comprises a first tube supported within a second tube, a first end of the first tube having an orifice therein and a first end of the second tube, adjacent the first end of the first tube, being closed, whereby a volatile liquid is passed through the first tube and expands as it exits the orifice into the second tube to cool the second tube.

5. An ionization chamber as in claim 2 wherein 'at least one of said first and second membranes is perforated.

6. An ionization chamber asin claim 2 wherein substantially identical electric potentials are applied to said first and second filaments and to said first and second electron focusing electrodes through said socket means for producing an electric field symmetrical about a line intermediate said filaments in a region between said electron focusing electrodes, including said ionization region.

7. An ionization chamber as in claim 2 wherein said ionization region bounds a volume of not more than 0.002 cubic inch.

8. An ionization chamber as in claim 2 including:

a capillary system; and

a sample inlet system coupled to the capillary system;

said sample insertion means comprising at least one capillary tube having an inner and outer end, and extending outwardly along a diameter of said ionization region, the inner end terminating immediately adjacent the perimeter of said ionization region, and the outer end being coupled with the capillary system. 

1. An ionization chamber comprising: a plurality of conductive members; a first and second coaxial, tubular electrode, the second tubular electrode being connected to at least one of said conductive members and each of the tubular electrodes having an electrically conductive membrance across one end, said ends being adjacent; an ionization region situated between the electrically conductive membranes and within a perimeter of one of the tubular electrodes; sample insertion means for introducing a sample to be ionized between said membranes; an arcuate filament connected to a plurality of said conductive members and disposed exterior said ionization region and in a plane lying intermediate said membranes; an electron focusing electrode connected to at least one of said conductive members, situated exterior said ionization region, and adjacent said filament for focusing electrons emitted by said filament into the ionization region for ionizing said sample; a third tubular electrode coaxial with the second electrode and connected to at least one of said conductive members for coacting with the second tubular electrode to form a lens for focusing ions exiting from the ionization region; an insulative support member for supporting saiD plurality of conductive members; means for supporting the first, second and third tubular electrodes, the filament, and the electron focusing electrode; and means for applying external signals via said conductive members to the filament and electrodes for causing electrons to bombard and ionize molecules of said sample and for propelling resulting ions through the electrically conductive membrance across one end of the second tubular electrode.
 2. An ionization chamber as in claim 1 wherein: a first and second arcuate filament are disposed opposite each other across a diameter of the ionization region and a first and a second electron focusing electrode are disposed adjacent said first and second filaments respectively; the means for supporting the second and third tubular electrodes, the first and second filaments, and the electron focusing electrodes are said conductive members; and said means for applying external signals includes socket means for engaging selected ones of said conductive members and said first tubular electrode.
 3. An ionization chamber as in claim 2 wherein said socket means includes heating and cooling means in a portion engaging said first tubular electrode for heating and cooling said chamber.
 4. An ionization chamber as in claim 3 wherein said cooling means comprises a first tube supported within a second tube, a first end of the first tube having an orifice therein and a first end of the second tube, adjacent the first end of the first tube, being closed, whereby a volatile liquid is passed through the first tube and expands as it exits the orifice into the second tube to cool the second tube.
 5. An ionization chamber as in claim 2 wherein at least one of said first and second membranes is perforated.
 6. An ionization chamber as in claim 2 wherein substantially identical electric potentials are applied to said first and second filaments and to said first and second electron focusing electrodes through said socket means for producing an electric field symmetrical about a line intermediate said filaments in a region between said electron focusing electrodes, including said ionization region.
 7. An ionization chamber as in claim 2 wherein said ionization region bounds a volume of not more than 0.002 cubic inch.
 8. An ionization chamber as in claim 2 including: a capillary system; and a sample inlet system coupled to the capillary system; said sample insertion means comprising at least one capillary tube having an inner and outer end, and extending outwardly along a diameter of said ionization region, the inner end terminating immediately adjacent the perimeter of said ionization region, and the outer end being coupled with the capillary system. 